Stimulation of muscarinic receptors in SK-N-SH human neuroblastoma cells leads to both phosphoinositide breakdown and stimulation of adenylate cyclase. This novel response is due to cross-talk between these two major signal transduction systems and appears to be mediated by a calcium/calmodulin-dependent process. As SK-N-SH cells express the m3 subtype muscarinic receptor, cross-talk may involve this receptor subtype. We have succeeded in solubilizing the D-1 dopamine receptor from both rat and bovine striatal membranes and are using a novel affinity chromatography procedure to purify it. We have shown that neuropeptide Y (NPY) inhabits adenylate cyclase in SK-N-MC human neuroblastoma cells. As inhibition is blocked by treating the cells with pertussis toxin, the NPY receptors are coupled through the inhibitory regulatory protein GI to adenylate cyclase. The availability of a continuous cell line will accelerate progress on characterizing NPY receptors and the mode of action of this important vasoconstrictor. Both human choriogonadotropin (hCG) and phorbol esters causes desensitization of the hCG receptor-coupled adenylate cyclase in murine Leydig tumor (MLTC-1) cells. Whereas phorbol esters activate protein kinase C, hCG does not. Thus, distinct mechanisms of desensitization appear to be involved. This has been confirmed in a cell-free system; hCG, but not phorbol esters cause desensitization of the membranes. Addition of purified protein kinase C to the membranes, however, does cause desensitization. MLTC-1 cells express messenger RNA for oxytocin and vasopressin. Stimulation of the cells with hCG leads to a decrease in their mRNA levels. This regulation is medicated by cyclic AMP as it can be an important source for these neurohypophysial hormones and their synthesis may be controlled by gonadotropins.